I've never taken the time to learn the "A" classifications. All of that was before my interest in the hobby started.

The modern stuff I'm aware of is grouped by engine size, or by its size relationship to a full size piloted plane.

The engine size classifications I'm most familiar with are .20, .40, .60, 1.20; and that refers to cubic inch displacement of the recommended two stroke glow engine.

To muddy this up even more, the recommended engines for a "40" size plane actually range from .40 to .55 cubic inch. Most of the popular two cycle engines in this range are very close to the same physical size and usually fit the same engine mount without modification. Weight varies some among the many brands and models available in this engine class.



A 1/4 scale cub could also be called a 25% Cub. Its size is theoretically 25 percent of the full size piloted airplane.
A 30% Extra is theoretically that percentage of the size of the full size piloted plane.

I write "theoretically" because most models that are small versions of an actual production plane have been redesigned some so they will fly well as models. The wing size may be perfectly 'scale' but the tail feathers are bigger than they should be; or some other variation.